I run both OS and apps from my SSD. Games, too (granted only game I have installed is Starcraft II). With a fixed pagefile size of 1GiB (only there because some apps work funky without a pagefile), I have 50GiB or so free space on my SSD (Intel X25-M G2 120GB). I mean really, what's the use of having an SSD if you have to wait on a slow hard drive when you're loading apps? Testing how quickly Windows boots?
I wouldn't worry about keeping it free, either, particularly with Sandforce. Sandforce "cheats" in that it compresses files before writing them to flash. That's how you get really fast read/writes with Sandforce. As long as you're not dealing with already compressed data (videos, mp3s, etc), Sandforce drives should have a lot of unused NAND flash than what's being reported in Windows.
Most drives already have 6.9~12.7% over-provisioning. If you want to increase over-provisioning do that when you format. For example, I have my Intel SSD formatted to 100GiB so that gives the controller an extra 11.8GiB reserved NAND for background maintenance, etc (total of 21.9% spare area).